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Inorganic Chemistry > Advanced Transition Metals > Flashcards

Advanced Transition Metals Flashcards

1
Q

What orbitals in an octahedral complex have the highest energy

A

Orbitals along the axis

Dx^2-y^2
Dz^2

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2
Q

Order of orbitals in a square planar complex

A

Dx^2-y^2
Dxy
Dz^2
Dxz dyz

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3
Q

Tetrahedral energy level order

A

Dxy dxz dyz

Dz^2 dx^2-y^2

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4
Q

Describe the MOs of an octahedral complex with six sigma donor ligands

A

6 ligand orbitals and 9 metal orbitals

6 bonding 6 anti bonding and 3 non bonding

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5
Q

The optimum number of valence electrons

A

Between 12 and 18 valence electrons

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6
Q

Rules of electron counting:

1) metal valence electrons
2) for ligands
3) oxidation state

A

1) group number is the number of valence electrons of the metal

2) L-type are 2 electron donors
X-type are 1 electron donors
Z-type are 0 electron donors (Lewis acids)

3) oxidation state of metal is the number of x type ligands

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7
Q

Describe transmetallation

A

Synthesis method for metal alkyl/aryls

Bigger difference in electronegativity increases rate of reaction but decreases the amount of selectivity

Stereochemistry, temperature and concentration also affect reaction

Reactants include RLi RMgX ZnR2 AlR3

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8
Q

Describe electrophilic attack as a synthesis route for metal alkys

A

R+ reagent
Nucleophilic metal compound required

Eg Na[Mn(CO)5] + MeI goes to
[Mn(Me)(CO)5] + NaI

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9
Q

Describe oxidative addition as a synthesis route for metal alkyls

A

Often observed in d8 sq planar complexes as they are most vulnerable along the perpendicular to the plane of the molecule

Literally just add things in e.g. MeI
They will be cis to each other

Normally 16 e- but can be 18 if prior de coordination occurs

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10
Q

Describe 2 methods of synthesis for fisher carbenes

A

1) nucleophilic attack at carbonyl ligand

Very versatile

A range of heteroatoms can be introduced

2) electrophilic abstraction from a metal alkyl complex

Using Me3SiCl

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11
Q

2 methods of synthesis for schronk metal complexes

A

1) alpha hydrogen elimination from a dialkyl precursor

The bulky ligand means the process is driven by steric hindrance

2) alpha deprotonation of a metal methyl complex

Use NaOMe

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12
Q

If a metal carbonyl has a lot of backbonding, where is it susceptible to attack

A

Electrophilic attack at oxygen

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13
Q

If a metal carbonyl has poor backbonding where is it susceptible to attack?

A

Nucleophilic attack at carbon

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14
Q

What type of ligands are alkenes

A

L-type

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15
Q

2 effects of backbonding to an alkene

A

Increases carbon carbon bond length

Reduces angles around carbon centres as sp3 contribution increases

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16
Q

Is a metal-alkenyl or metallacyclo structure more reactive?

A

Alkene is d+ and therefore is susceptible to nucleophilic attack

Increased back bonding in the cyclic compound reduces the charge so the metallocyloalkane is less reactive

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17
Q

A metal carbon sp3 bond describes what type of compound

A

Metal alkyl including cycles

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18
Q

Metal carbon sp2 bonds are found in what 3 types:

A

Metal-aryl
Metal-alkenyl
Metal-acyl

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19
Q

Metal carbon(sp) bonds are found in which 3?

A

Metal carbonyl
Metal alkynyl
Metal isonytryls

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20
Q

Oxidative addition as a main reaction:

A

Oxidation number, coordination number and VE count all increase by 2

Metals with low oxidation states

Can have concerted, stepwise, radical or ionic mechanisms

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21
Q

Reductive elimination

A

Oxidation state, coordination number and VE count all decrease (-2)

Metals in medium or high oxidation states

Non polar 3 centre transition state

22
Q

Sigma metathesis

A

Typical of early d-block elements in high oxidation states

No change in oxidation state

4 membered cyclic transition state

Concerted process

23
Q

Ligand substitution

A

D electron count, coordination and d electron count remain unchanged

Can be associative or dissociative

24
Q

Migratory insertion

1,2 and 1,1 insertion

A

No change in oxidation state

Ligands must be cos

1,2 insertion: an unsaturated L type ligand is inserted into a sigma M-X bond after the unsaturated ligand has associated to the metal

1,1 insertion: typical of a carbonyl complex, an x ligand migrates to a coordinated co to form an acyl, leaving one vacant site

25
Hydride elimination reactions
Reverse of insertion No change in oxidation state Alkene often eliminated
26
Nucleophilic attack of coordinated ligand
Unsaturated organic compounds are activated towards nucleophiles when coordinated to electron deficient metals Causes elimination of alkene
27
Electrophilic abstraction
Alkyl or hydride ligands might be abstracted by strong electrophiles such as B(C6F5)3
28
Coordinated alkane spectra: Proton NMR
Do not differ drastically Dependant in metal, oxidation state and other ligands
29
Coordinated alkane spectra C NMR:
Both positive and negative chemical shifts For early metals the signals are more downfield due to deshielding (the opposite is true for late transition metals)
30
Coordinated arene C NMR
For metal aryls the sigma bound ipso carbon tends to be more deshielded than the corresponding arene Arene substituents also affect chemical shift, depending on donating and withdrawing effects
31
Increasing kinetic stability of M-C bonds
Thermodynamically stable but there are many reaction pathways so kinetically unstable Coordinatively saturated complexes with no Kabila ligands are more stable Increasing steric hindrance with bulky ligands also increases stability UNLESS PREVENTING ELIMINATION
32
Beta hydride elimination reactions and 3 things you need:
Mainly metal alkyls Driving force is the formation of a stronger M-H bond and generation of an alkene (reduces the unsaturation of the metal) Need: B-hydrogens Vacant sites M-c-c-h syn-coplanar arrangement possible
33
How can beta hydride elimination reactions be avoided:
Using ligands with no b-hydrogens Using metals with no empty coordination sites Small metallacycles show higher stability as there is no syn-coplanar orientation. The smaller the cycle the more stable it is but they still react when heated Double bonds to bridgehead carbons are unfavourable
34
Alpha hydride elimination reactions
Observed when B-H elimination is not possible Mo and Ta are most likely to react in this way Alpha hydrogens must be available Free coordination site cis to CH3 Product is very reactive and will proceed to react with any nucleophile All elimination favoured more as steric hindrance around metal increases
35
Benzyne formation
Not common due to instability of products CH bond activation process High reaction temps and increasing steric hindrance around a metal Can go on to form many organic compounds with stereochemical control due to insertion step Alkyne lumo is very low therefore acts as an electrophile
36
Reductive elimination
Reverse of oxidative addition Two groups in a cis arrangement More common for alkyl and square planar complexes Prevented by strongly chelating ligands and restrictive geometries Common for biaryl decomposition, where the reactant molecule can be free or bound to the metal centre
37
Cross coupling catalysts:
1) classics [Pd(PPh3)4] , [Pd2(dba)3] or Pd(OAc)2 + PPh3 Palladium (II) can be used as a pre catalyst and be reduced in situ 2) More sophisticated ligands leave very high turn over frequency and turn over number under mild conditions
38
How to promote cross coupling:
Strong sigma donating ligands are essential as they facilitate the oxidative addition step and help avoid catalyst deactivation by stabilising low coordinated palladium centres Steric hindrance also has beneficial effects: Promotes reductive elimination Offers kinetic stability of low coordinated centres
39
Hydrogen activation via oxidative addition
Either dihydride homolytic activation or monohydride heteolytic activation Reduced pKa of H2 to between 0 and 20 from 35
40
Agnostic interactions:
3c-2e bonding in electron deficient metals Can be alpha or beta if hydrogens are available Can lead to cyclometallation
41
Characterisation of agostic interactions
1) H atoms not detected by X-ray crystallography so neutron diffraction must be used 2) proton NMR is often unclear and requires the solid state 3) IR has lower C-H stretching frequencies but they are hard to distinguish from other signals
42
Characterisation of Metallicycles
1) H NMR Signals at low frequencies (-5 to -45) Non equivalent hydrides will couple Coupling to phosphines can determine structure 2) IR M-H stretching frequencies in the range 1500-2200 cm^-1 but usually weak 3) clear X-ray distortions observed
43
Why is functionalisation of C-H bonds a problem?
1) Alkanes are inert due to strong localised bonding 2) when alkanes react it is at high temperatures or with very reactive compounds, which is unselective 3) formed products are always more reactive than the alkane which can lead to overreaction
44
C-H oxidative addition
Thermodynamically unfavourable due to strength of M-H vs M-C bonds Products are prone to reductive elimination Most activation is aromatic or involves agnostic interactions
45
Cyclometallation and aryl ligands
Activation of ortho substituents is very easy The outcomes are either 1) formation of a metal hydride with a higher oxidation state 2) cleavage of an anionic ligand
46
Intermolecular Oxidative addition
1) C-H activation of arenes: Good synthesis strategy of metal aryl complexes 2) can also allow the C-H activation of alkanes
47
Sigma bond metathesis and C-H activation
Typical of early metals with a d0 configuration (as no OA can occur) Concerted reaction via 4 centre TS LM-R + M'-R' to LM-R' + M'-R From d2-d10 both OA and SBM are permitted and often the mechanism can't be confirmed Can also be used for H2 and in rare cases for C-C bonds
48
Metaloradical activation
Dimetallic complexes (typically Rh) exist in equilibrium with the monomeric complexes These react with R-H to form two new complexes Methane is the most active compound for this Kinetic control- when toluene is used there is no aromatic C-H activation
49
Electrophilic Activation: The two different mechanisms
Can either proceed by: 1) ligand as internal base- M coordinates to R-H bond then the bong breaks, with metal coordinating to R and XH. XH then dissociates 2) preactivation with external base M has 2+ charge and coordinates to the RH bond, then an external base reacts with H leading to MR and HX IN ONE STEP
50
Electrophilic activation:
Displacement of H by a metal: [M+] + RH > [M]R + H Typical of cationic complexes of strongly electrophilic metals in normal to high ox states (Pd(ll) Pt(ll) Pt(IV) Hg(ll) and Ti (III) Often carried out in strongly polar media such as water or strong acids

Inorganic Chemistry (1 decks)

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